Lifting the reat of the bonnett up is prolly upsetting the airflow through the engine bay. notice how the top edge of the fire wall has a rubber seal on it that the bonnett sits on?? My guess is the airflow is designed to flow through the radiator/front of the car then pressurise the engine bay to a degree and push the hot air out underneath the car. ever wondered why the plastic undertray doesn't go all the way to the firewall?? that's where the air escapes the engine bay. Plus the undertray extends about a foot or so past the back edge of the radiator which would create a low pressure area but with the fans on the backside of the radiator pulling air through it then pushes the air straight towards the engine block and subsequently down under the car. With a low pressure area behind the cooling fans, when they switch on they wil pull a heap more air through because of the pressure difference. hope this kinda makes sense as it is hard to explain without actually showing it on a car. Things you learn working on open wheel race cars!!